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Resistance of Enterococcus faecium to neutrophil-mediated phagocytosis
R C Arduino1, K Jacques-Palaz, B E Murray
1Department of Internal Medicine, University of Texas Medical School at Houston 77030.
Abstract:
During a previous study of the opsonic requirements for neutrophil (polymorphonuclear leukocyte [PMN])-mediated killing of enterococci, we identified two strains of Enterococcus faecium (TX0015 and TX0016) that were resistant to PMN-mediated killing. To better define the mechanism of this resistance, we examined phagocytosis with a fluorescence assay and found that TX0016 was completely resistant to phagocytosis by PMNs; this finding was confirmed by electron microscopy. Examination of multiple strains of enterococci revealed that all 20 strains of Enterococcus faecalis tested were readily phagocytosed (mean, 18 intracellular organisms per PMN; range, 7 to 28). In contrast, only 13 (50%) of 26 strains of E. faecium tested were susceptible to phagocytosis (> or = 7 organisms per PMN); the other 13 strains showed < or = 3 organisms per PMN. Enterococcus casseliflavus ATCC 25788 and one strain of Enterococcus hirae were also resistant to phagocytosis, while two strains of Enterococcus durans, Enterococcus mundtii ATCC 43186, and one strain each of Enterococcus raffinosus and Enterococcus solitarius were readily phagocytosed. Exposure of E. faecium TX0016 to sodium periodate, but not to the protease trypsin or pronase or to phospholipase C, eliminated resistance to phagocytosis. Sialic acid, a common periodate-sensitive structure used by microorganisms to resist opsonization, could not be demonstrated in E. faecium TX0016 by the thiobarbituric acid method, nor was phagocytosis of TX0016 altered by neuraminidase treatment. This study suggests that there is a difference in susceptibility to phagocytosis by PMNs between different species of enterococci and that a carbohydrate-containing moiety which is not sialic acid may be involved in the resistance of E. faecium TX0016 to phagocytosis.
Insights
Certain Enterococcus faecium strains resist phagocytosis by neutrophils (PMNs). This resistance is linked to a non-sialic acid component, impacting bacterial killing and suggesting species-specific immune evasion strategies.
Area of Science:
- Microbiology
- Immunology
- Bacterial Pathogenesis
Background:
- Neutrophils (PMNs) are crucial for killing bacteria, including enterococci.
- Previous studies identified Enterococcus faecium strains resistant to PMN-mediated killing.
- Understanding resistance mechanisms is vital for treating enterococcal infections.
Purpose of the Study:
- To investigate the mechanism of phagocytosis resistance in Enterococcus faecium TX0016.
- To compare phagocytosis susceptibility across different enterococcal species and strains.
- To identify bacterial components involved in evading PMN-mediated clearance.
Main Methods:
- Phagocytosis assays using fluorescence microscopy to quantify intracellular bacteria within PMNs.
- Electron microscopy to confirm resistance to phagocytosis.
- Treatment of bacteria with enzymes (proteases, phospholipase C) and chemicals (sodium periodate, neuraminidase) to assess their effect on phagocytosis.
- Chemical assays (thiobarbituric acid) to detect sialic acid.
Main Results:
- Enterococcus faecium TX0016 was completely resistant to phagocytosis by PMNs.
- Significant variation in phagocytosis susceptibility was observed among enterococcal species; E. faecalis strains were readily phagocytosed, while only 50% of E. faecium strains were susceptible.
- Exposure of TX0016 to sodium periodate eliminated its resistance to phagocytosis, suggesting a carbohydrate-containing surface structure.
- Sialic acid was not detected in TX0016, and neuraminidase treatment did not alter phagocytosis, ruling out sialic acid as the resistance factor.
Conclusions:
- Phagocytosis susceptibility varies significantly between enterococcal species and strains.
- A periodate-sensitive, non-sialic acid carbohydrate moiety on E. faecium TX0016 likely mediates resistance to PMN phagocytosis.
- This finding highlights novel immune evasion strategies employed by enterococci.